The invention relates to foamed metal preformed bodies and a method with which a metal foam material of a preformed body is foamed during fabrication of the preformed body.
It is especially in lightweight structural engineering that there has always been a problem in joining individual structuresxe2x80x94termed preformed bodies in the followingxe2x80x94in keeping with the requirements of the later application, not least due to the demand for low weight while at the same time ensuring high mechanical loading capacity and stability at low cost. This is also basically a problem in other fields, for instance, in shipbuilding where particularly large-area preformed bodies, for example hull segments, need to be joined to each other, although here too, there is always the requirement for the smallest deadweight possible while assuring high mechanical loading capacity of such preformed bodies and the composite bodies composed thereof.
One possibility of saving weight is to make use of preformed bodies of foamed metal foam material or fractions of such materials. The pore structure of the frothed or foamed foam metal results in a frothed or foamed metal preformed body having a lower weight than a solid metal preformed body while still assuring high mechanical loading capacity. Aside from this, metal foam material features a whole series of additional advantageous properties, such as, for example, high shock absorption, noise absorption, as well as a reduced heat conductivity and electrical conductivity as compared to the solid material. To achieve these advantageous properties the foamed material needs to have a foam or porous structure as uniform as possible.
Metal foam materials and the semi-finished products and preformed bodies produced therefrom are known, for example, from DE 41 01 630 C2, DE 43 18 540 A1, DE 44 16 371 A1, DE 44 26 627 A1, DE 196 12 781 C1 and DE 197 17 894 A1. Proposed in DE 43 18 540 A1 is the use of metal foam material in automobile production in which body panels are employed as foamed metal preformed bodies comprising a solid metal skin and a foamed layer of metal foam. For securing fitted items to these body panels mention is made of self-cutting screws and dowel-type fasteners. DE 196 12 781 C 1 likewise relates to preformed bodies for motor vehicles. In the publication xe2x80x9cMetallschxc3xa4ume 1997xe2x80x9d, MIT Bremen, in the article entitled xe2x80x9cJoining of Aluminum Foamsxe2x80x9d by N. Sedliakova et. al., screw fasteners, soldered, cemented and welded joints as well as the use of foamed fasteners are described as methods for joining foamed metal preformed bodies. These are the accepted methods for joining preformed bodies, the advantages and disadvantages of which are well known in engineering.
An object of the invention is to provide a foamed metal preformed body having a structure of the foamed metal foam material which is favorable for the intended purpose of the preformed body. For the majority of purposes of the preformed body, it is desirable that the porous structure of the foamed metal is as uniform as possible.
This object is achieved by the subject matter as set forth in the independent claims.
The invention relates to a preformed body comprising foamed metal foam material cited in the following as a foamed metal preformed body due to this property.
The foamed material may totally fill a preformed body configured hollow.
The foamed material may be sited also only in one or more zones of the preformed body, where it fills out cavities, or it may also reinforce only one structural part of the preformed body or join structural parts.
In accordance with the invention, the metal foam material is thermally treated internally during foaming. A thermal treatment means is conducted through the metal foam material. The thermal treatment means may be arranged so that is only comes into immediate contact with the material to be foamed when actual foaming has commenced. However, the thermal treatment means can also already come into direct contact with the metal foam material before the material is foamed to, for example, already initiate foaming of the material. Foaming may be supported or even first made possible when, for example, introducing heat to the zone of the thermal treatment means is not possible due to the low thermal conductivity of the foamed metal or possible only to an inadequate degree for foaming. The thermal treatment means may also serve merely as a coolant, or in addition to heating.
The thermal treatment means involved is preferably a flow conduit for communicating, or also merely for introducing, a thermal treatment fluid through or into the zone of the preformed body in which the metal foam material is located. The thermal treatment means may, however, also be formed in principle by some other means such as, for example, an electrically conducting wire of a resistance heating element if only internal heating is to be implemented during foaming. By means of a flow conduit, heating or cooling is optionally possible to advantage within the structure of the metal foam material. With such a thermal treatment means, the metal foam material is heated during its foaming or cooled to discontinue foaming or subsequent to foaming. Also applicable is a combination of two or all three thermal treatment procedures. It is particularly of advantage when the complete foaming procedure is controlled by thermal treatment. The foamed material is heated internally for foaming and, if necessary, maintained at a temperature by means of internal thermal treatment, which, in contrast to external heating, is very accurately adjustable and uniform throughout the foamed metal, and cooled for consolidation.
Internal thermal treatment permits highly accurate thermal treatment of a localized zone in which metal foam material is arranged. Zones are thus attained in which, due to the shape of the preformed body and the amounts of foamed metal located therein, cold or hot spots could materialize should heating or cooling be done merely externally. Directly adjoining zones may even be heated and cooled simultaneously. Zones may be protected from excess heat entry. Heating may be done internally and cooling externally, and vice-versa. Accordingly, the choice of materials possible for the dimensionally stable structural parts during foaming is greater. Different foam materials may even be combined in that a material having a low foaming temperature due to the internal thermal treatment is specifically protected against excessive heating of an adjoining material having a higher foaming temperature.
The result of the above is that foaming any shape of preformed body is made possible. By means of internal thermal treatment, preformed bodies of any internal geometry and also any outer dimensions may be foamed, for which, otherwise, special ovens would need to be built. Such preformed bodies may be foamed fully or partly as a whole in a single heating operation.
A preformed body foamed with foamed metal in accordance with the invention is characterized by a thermal treatment means, preferably at least one flow conduit for a thermal treatment fluid, being guided through the foamed structure. In one preferred embodiment, thermal treatment fluid is passed through in counterflow. For this purpose, two flow conduits, through which the thermal treatment fluid is passed in counterflow, may be oriented closely spaced in parallel or in direct thermal conductive contact with each other. In both flow conduits, thermal treatment fluid is introduced at the desired temperature and passed through in counterflow. Along the pair of flow conduits, the thermal treatment temperature materializes particularly uniform. Instead of a thermal treatment fluid passing through flow conduits separately in flow, it may also be passed through the two flow conduits in sequence. In both flow guidance arrangements, one of the two flow conduits may surround the other.
Cooling the foamed metal internally during its foaming prevents uncontrolled further foaming of the foam material. The foaming procedure is purposefully concluded; the foaming action being stopped as soon as the desired homogeneous foamed structure has materialized, thus preventing individual pores in the foam from being expanded to such an extent that they would ultimately collapse. Without the controlled internal thermal treatment in accordance with the invention, in particular inner cooling, there would always be the risk of the walls of the pores becoming so thin, due to the expansion, that they are no longer stable and xe2x80x9cweepagexe2x80x9d of the remaining foamed metal would occur, resulting in large voids materializing in the preformed body. This effect being known as the xe2x80x9cdrainagexe2x80x9d effect. In accordance with the invention, this is prevented by the controlled internal thermal treatment.
In the foamed zone, several of the pairs of flow conduits as described above may be arranged. Indeed, combinations of both single and pairs of flow conduits may also be formed.
Due to this nested flow conduit configuration and guidance of the thermal treatment means through the same in counterflow, the temperature of the thermal treatment means and, thus ultimately, of the foamed metal is set particularly uniformly over a long length of flow conduit. In accordance with one particularly preferred embodiment of nested flow conduits, one outer flow conduit accommodating an inner flow conduit is closed off at one end which may be located in the foamed metal. For foaming, the hot thermal treatment fluid is introduced into the inner flow conduit, through which it flows, and emerges therefrom at the closed end of the outer flow conduit to enter therein before then flowing in the intermediate space between the inner and outer flow conduit back to a thermal treatment means. In this arrangement, a compensation in the heat occurs between the thermal treatment fluid in the inner flow conduit and the return flow of thermal treatment fluid. This results in a particularly uniform temperature of the return flow of the thermal treatment fluid over the full length of the flow conduit.
Instead of the blind thermal treatment means as described above, in which the thermal treatment fluid is led in and out at the same end, the thermal treatment fluid may also be led in and out at the opposite ends of these flow conduits in each case. This is only possible, however, when the inner and outer flow conduits are brought out at both ends from the metal foam zone. Accordingly, in the following, reference is made simply in general to an outer flow of fluid instead of a return flow of fluid.
The inner flow conduit is preferably located over its full length centrally spaced away from the outer flow conduit. This may be achieved by means of spacers arranged between the inner flow conduit and the outer flow conduit. These spacers are made of a heat resistant material, for instance of ceramics.
Preferably, the outer flow of the thermal treatment fluid flowing is turbulent, as a result of which the heat conducted from the inner flow conduit into the outer flowing thermal treatment fluid and from the outer flowing thermal treatment fluid to the foamed metal is enhanced as compared to that of a laminar flow. The spacers may be configured accordingly as the means for producing turbulence. It is particularly preferred to produce the turbulent outer flow by means of a tape, wound spirally around the inner flow conduit.
In the preferred embodiment, such a tape simultaneously serves as a spacer, i.e. no further spacers are needed in addition thereto. Preferably, the tape is configured as a woven tape, more particularly of heat resistant fibers such as e.g. glass, ceramic or carbon fibers. Due to the tape being wound spirally around the inner flow conduit, the thermal treatment fluid is, likewise, guided and swirled spirally in the intermediate space between the inner and outer flow conduits.
If the flow conduits are formed by tubes, these tubes are at the foaming temperatures of e.g. 500 to 800xc2x0 C. in the case of aluminum foam material, above the softening temperature of a whole series of metallic materials which, where aluminum is concerned, is approximately 180xc2x0 C. When aluminum is selected as the material for the flow conduit, the foaming temperature of the aluminum foam is near to or already above the solidus temperature of aluminum of approximately 660xc2x0 C. Accordingly, there is thus the risk of the flow conduit(s) being squeezed in the metal foam zone due to the material foaming there. Preferably, the flow conduit or several flow conduits is/are made of a heat resistant material, preference being given to glass, ceramics, carbon and/or other heat resistant materials, especially fiber materials. To improve the thermal conductivity of the particularly heat resistant material, fibers of another material are worked into the material of the flow conduits, the solidus temperature of this other material being significantly above that of the metal foam material used. In the event of aluminum foam being used, it is preferred that these worked-in heat conducting fibers are fibers of graphite, molybdenum and/or tungsten. The melting temperatures of these materials are significantly above 2000xc2x0 C. and the thermal conductivity at 20xc2x0 C. amounts to at least 130 W/mK. Instead of the materials cited by way of example, other materials having a sufficiently high melting temperature and an adequately high thermal conductivity, especially having a melting temperature exceeding 2000xc2x0 C. and a thermal conductivity of at least 130 W/mK may also be employed. Provided that a heat resistant and pressure resistant material is concerned which simultaneously also has a good thermal conductivity and is compatible with the material to be foamed, the flow conduit(s) may also be entirely made of such a material; graphite being one example of such a material.
In the event of using nested flow conduits, the outer flow conduit may remain in the metal foam structure upon completion of the foaming procedure. The remaining outer flow conduit improves the mechanical properties of the preformed body, especially when the internal thermal treatment is undertaken in a jointing zone of several preformed bodies. The inner flow conduit as well as the spacers, or the fiber tape may be preferably removed from the outer flow conduit to be available for repeat use. The inner flow conduit and the spacers, or the fiber tape replacing the spacers, may, likewise, remain in the metal foam structure, however. By preferably filling out the cavity, surrounded by the outer flow conduit, with plastics material, the preformed body or the composite zone may be further reinforced mechanically. In all of the cases cited a fiber-reinforced metal preformed body materializes. It is particularly preferred to make use of a flow conduit or nested flow conduits made of carbon fiber material, thus resulting in carbon-reinforced foamed metal preformed bodies.
A thermal treatment system comprises preferably an internal combustion engine with a turbocharger, the exhaust air being employed as the heating fluid, the intake air to the charger being used as the cooling fluid. The force driving the engine may be used to generate electricity.
The thermal treatment means may be used during repair work to reduce the heat introduced into the foamed preformed body to a safe degree, this being particularly of advantage when. repairing after foaming a joint as preferred.
The thermal treatment means also forms a mechanical reinforcement of the foamed preformed body and is arranged to advantage also in a way optimum in this respect, thus resulting also in a fiber composite body in which a thermal treatment means forms a fiber.
The flow conduit may be perforated to bring the thermal treatment means into direct contact with the foam material.
In another preferred embodiment, the metal foam material is charged from within by a fluid, preferably an inert gas or reaction gas by means of which the oxidative effect of a foaming agent contained in the foamable metal foam material is reduced or increased. A perforated flow conduit, arranged for this purpose in the jointing zone, may be formed by the aforementioned thermal treatment flow conduit; although a separate further flow conduit having small openings, i.e. a perforated flow conduit may be incorporated. In the case of nested flow conduits, the outer flow conduit, of course, would be perforated.
The thermal treatment fluid and the fluid for controlling an oxidation may be formed by one and the same fluid.
A preformed body preferably used is fabricated from a slab of laminated material comprising at least one layer of a foamable metal foam material and an adjoining skin of a metallurgically compatible solid metal material. From such a semi-finished product, a preformed body in accordance with the invention is produced by known forming methods, more particularly cold forming. Typically, both the foamed material and the solid material is formed by the same metal or the same metal alloy, for example, foamable aluminum foam and solid aluminum. However, any compatible unlike pairing of materials is just as usable for the purposes of the invention, the foaming temperature of the foam material being preferably below the melting temperature of the solid material. The foam material of one or more preformed bodies may be foamed even prior to jointing. One or more of the preformed bodies to be joined may also consist only of the metal foam material.
However, the invention is not restricted to application with preformed bodies containing foam metal materials, it also being applicable to advantage with metallic preformed bodies having no foam material, for example with conventional sheets of metal or other sections or shells. In addition, preformed bodies obtained by sintering or casting, for example, may be involved. The invention is, although preferred, not restricted exclusively to metallic preformed bodies. In principle, any kind of preformed body may be foamed by means of the internal thermal treatment in accordance with the invention, as long as these preformed bodies are not ruined by the foaming temperature of the foam material used in each case; preferably they should remain dimensionally stable at the foaming temperature. As already mentioned, the invention provides in this respect additional freedom, also as regards selecting the material.
Connected to the thermal treatment means is preferably at least one structured thermal treatment sheeting increasing the outer surface area of the thermal treatment means. The structured thermal treatment sheeting protrudes from the thermal treatment means into the foamed metal. Between the structured thermal treatment sheeting and the thermal treatment means, an optimal thermal conductivity should exist. These additional thermal treatment surface areas of the structured thermal treatment sheeting act as cooling and/or heating fins, In one preferred embodiment, one such structured thermal treatment sheeting is formed by several surface areas oriented at an angle to each other. Preferably, each two adjoining thermal treatment surface areas are oriented at an angle to each other in the range 90 to 140xc2x0, preferably 110 to 130xc2x0. It is particularly preferred that these surface areas comprise in their edge portion, or in an imaginary edge portion should the surface areas fail to come up to the imaginary edge, an angle of 120xc2x0 relative to each other or almost 120xc2x0. This enables the liquid metal material, defining the individual pores or foam bubbles during foaming, to be in particularly good contact with the additional thermal treatment surface areas so that the thermal conductivity in the structure of the foamed metal is particularly good.
In the first embodiment, such additional thermal treatment surface areas are configured as structured metal sheet formations. For this purpose, flat sheeting having a pocket or dished structure, produced by stamping for instance, may be provided. In another preferred embodiment, zig-zag lengths of sheet metal strip are employed as additional thermal treatment surface areas. Both embodiments comprise two surface areas merging at an edge or their imaginary elongations at an angle as described above. The one pocket has a size corresponding to a third or two-thirds of the expected average size of a single foam bubble or pore. Preferably, the volume of the single pocket is half the size of the expected average volume of a foam bubble or pore.
A thermal treatment flow conduit is maintained in the desired position preferably by means of spacers in the preformed body to be foamed. Preferably, a spacer forms an additional thermal treatment surface area.
Internal thermal treatment is employed to advantage in a jointing method in which preformed bodies are compacted into a solid composite body with the aid of a metal foam material. For this purpose, in a first step the preformed bodies to be joined are located relative to each other in their positions as desired for the composite, preferably by being clamped to each other. In their jointing zone, use is made of a jointing clamp to define a space, more particularly a cavity, to thus form an encasement in which foamable metal foam material is arranged. The metal foam material may be brought into the space encased by the jointing clamp either prior to, during, or after joint clamping. In a second step, the foamable metal foam material is foamed, this being preferably done by heating it to the foaming temperature of the metal foam material. Due to the increase in volume of the metal foam material involved in foaming and it being limited by the sufficiently solid encasement, the preformed bodies are compacted into a composite body. The jointing clamp itself is understood as a preformed body according to the invention.
In a first preferred embodiment, a prefabricated, separate jointing clamp is employed as the jointing clamp. This separate jointing clamp is placed on the preformed bodies to be joined, the preformed bodies having been preferably arranged matching each other in the shape of the later composite, by it clasping the preformed bodies or at least parts thereof. In this arrangement, the jointing clamp may already clampingly locate the preformed bodies to be joined together in their desired position for the composite relative to each other. A non-clamping application of the jointing clamp is likewise possible, however.
In accordance with a second preferred embodiment, the jointing clamp is formed by the corresponding configuration of the preformed bodies themselves so that affixing a separate jointing part may be eliminated. In this case, the jointing clamp is an integral component of one of the preformed bodies or is formed in the cooperation of the preformed bodies.
By inserting a suitable supplemental section, foam material may be brought into the foaming zone optimized in quantity and form as regards the foaming procedure and the subsequently foamed preformed body. In this way, should the preformed body not contain any foamable metal foam material itself in the foaming zone, foam material is furnished at least by a supplemental section.